> > Yes. Do we have any concrete use case where the subscriber is an older > > version, in the first place? > > > > As per my understanding, it is mostly due to the reason that it can > work today. Today, during an off-list discussion with Jonathan on this > point, he pointed me to a similar incompatibility in MySQL > replication. See the "SQL incompatibilities" section in doc[1]. Also, > please note that this applies not only to initial sync but also to > schema sync during replication. I don't think it would be feasible to > keep such cross-version compatibility for DDL replication.
I think it's possible to make DDL replication cross-version compatible, by making the DDL deparser version-aware: the deparsed JSON blob can have a PG version in it, and the destination server can process the versioned JSON blob by transforming anything incompatible according to the original version and its own version. Regards, Zane
